1. Technical Field
The present invention relates to electronic devices and, more particularly, to an improved pulse width modulator circuit.
2. Description of the Prior Art
Pulse width modulation (PWM), sometimes referred to as pulse duration modulation (PDM), is a common method of amplifying an audio signal for high efficiency amplification. The technique is linear for large signals and can be used to achieve a very high power gain. The pulse width modulation is achieved by sampling an input audio signal relative to a symmetrical ramp signal. An output pulse width modulated signal is produced with the length of each pulse being indicative of the amplitude of the input signal over one cycle of the high frequency ramp signal. The pulses drive a switching device that amplifies the modulated pulse width signal. An output signal is then applied to a reconstruction filter to recover the amplified output signal.
Pulse width modulation varies the width of the output drive pulse based on the amplitude of the audio input signal. The width is longer for a higher level and shorter for a lower level. If the input signal level is zero, the pulse width will be represented as zero. Due to the practical limits of the switching speed and propagation delay of the transistor employed as the switch, pulse width distortion becomes noticeable as the pulse width becomes narrow. As the pulse width approaches zero, pulses become so narrow that the pulses are missed. This contributes to linear distortion and creates a sudden drop-out effect in the audio signal.
In an AM (amplitude modulated) transmitter system employed as in radio broadcasting, a pulse width modulator is cascaded with an RF amplifier, serving as a main power amplifier stage.
FIG. 1 illustrates a prior art pulse width modulation system 10. An input signal, as from an audio source 12, is supplied to a pulse generator control block 14 along with a symmetrical ramp signal from a ramp source 16. The pulse generator compares the two signals and outputs a voltage high signal during each period when the input signal from the audio source 12 exceeds the ramp signal. For each cycle of the ramp signal, the pulse generator 14 creates a pulse of a width dependent upon the signal amplitude during that cycle. This is known as the duty cycle of the signal, and the length of an individual pulse is described as a percentage of this duty cycle.
The pulses are referred to as drive pulses and these drive pulses are supplied as turn-on signals to a switch 18, which may take the form of a semi-conductor device known as a MOSFET. This switch, when turned on, passes a DC signal obtained from a DC voltage supply 20. This voltage pulse is then supplied to a load that includes a diode 22 and a reconstruction filter 24 and an output amplifier 26 which is driven by drive pulses in synchronism with those supplied to the switch 18. This places an amplified, but filtered, output signal across the primary winding of a transformer T1 having its secondary winding applied to a load L by way of an inductor-capacitor circuit 28.
FIG. 2 illustrates a series of graphs showing voltage with respect to time of various signals employed in the operation of the prior art circuit of FIG. 1. These include graphs 30, 32, 34, 36 and 38.
Graph 30 illustrates a ramp signal 40 from the ramp source 16 combining with the audio input signal 42 obtained from the audio source 12. This combination results in the pulse width drive pulses 44 shown in waveform 32. These pulses have widths that vary with the amplitude of the audio input signal 42.
The drive pulses 44 are supplied to the switch 18 providing a train of output pulses 46 as shown in the waveform 34. Because some of the drive pulses 44 are too narrow with respect to the turn-on/turn-off characteristics of the transistor 18, some of the pulses are missing. The PWM output pulses 46 are supplied to the reconstruction filter 24 to provide a reconstructed signal 48 as shown in waveform 36. This waveform, however, has a portion 50 which is distorted because of the missing pulses in the PWM output drive pulses 46. Consequently, the resulting RF output 52, as shown in waveform 38, is distorted at location 54 in this waveform.